I love that it provides me with an easy way of monitoring my sites performance, whether its in development or live environment! It also allows us access to all our data so we can track trends for future improvements etc.
There are no downsides really as far as i am aware at this time but if you're looking into something like this then please do try out both products side by side before making any decisions about purchasing them together vs separately. We use testo to monitor several websites within our business which helps keep everything running smoothly.